Tunable lasers have applications in a wide variety of fields, including optical communications, optical fiber sensors, and spectroscopy. The availability of high performance erbium-doped fiber (EDF) amplifiers and lasers pumped with modest power now offers the possibility, with a suitable tuning element, of designing compact, diode-laser pumped, electrically tunable fiber lasers for the wavelengths near 1.5 .mu.m, which are of interest for optical fiber communications. Erbium doped fiber amplifiers and lasers are described, for example by W. L. Barnes, P. R. Morkel, L. Reekie and D. N. Payne, "High-Quantum-Efficiency Er.sup.3+ Fiber Lasers Pumped at 980 nm", Opt. Lett, Vol. 14, p. 1002 (1989), by E. Desurvire, C. R. Giles, J. R. Simpson and J. L. Zyskind, "Efficient Erbium-Doped Fiber Amplifier at a 1.53-.mu.m Wavelength with a High Output Saturation Power", Opt. Lett., Vol. 14, p. 1266 (1989), and by J. L. Zyskind, D. J. DiGiovanni, J. W. Sulhoff, P. C. Becker and C. H. Brito Cruz, "High Performance Erbium-Doped Fiber Amplifier Pumped at 1.48 .mu.m and 0.97 .mu.m", in Technical Digest on Optical Amplifiers and Their Applications, 1990 (Optical Society of America, Washington, D.C., 1990), Vol. 13, p. PdP6-1.
A practical tunable laser should be compact and of simple design. Its pump requirements should be modest, and the laser should be electrically tunable. Prior art ring lasers have involved hybrid technologies and used tuning elements which were bulky and had strong polarization dependence requiring complicated design and complicated operation. These prior art devices also had high losses that result in low slope efficiency and thus high pump requirements. Accordingly, to date, a ring laser with a satisfactory tuning element has not been described.